A conventionally known optical module used for transmission or reception of an optical signal is configured such that a polymer waveguide film (external waveguide substrate) having a waveguide (optical waveguide) with a core formed therein is bonded to a submount (referred to as a mounting substrate in this invention) carrying one or more optical elements (e.g., a light-emitting element and/or a light-detecting element) by adhesive with the polymer waveguide film positioned above the optical element(s) of the submount, whereby light emitted from the optical element is coupled to an incident end surface of the waveguide (refer to Patent Document 1).    Patent Document 1: Japanese Laid-open Patent Application No. 2006-243467
If, however, the polymer waveguide film is to be bonded to the submount in the aforementioned fashion, it is essential that the polymer waveguide film be precisely shaped to prescribed external dimensions, for instance, so that a position offset between the polymer waveguide film and the submount would never occur. It is practically so difficult to manufacture optical modules that way. Furthermore, even if the polymer waveguide film is manufactured with high dimensional accuracy and placed at a specified position on the submount, the polymer waveguide film might be displaced from the specified position when adhesive is filled into a gap between the polymer waveguide film and the submount, potentially causing an offset of optical axes of the polymer waveguide film and the submount and a resultant reduction in optical coupling efficiency.
There is also a case where a core of a waveguide is displaced relative to an outer shape of the polymer waveguide film. In this case, the core of the waveguide is displaced even if the polymer waveguide film is placed at the specified position on the submount. This could potentially cause an optical axis offset between the polymer waveguide film and the submount and a resultant reduction in optical coupling efficiency.
Also, since the polymer waveguide film is positioned above optical elements, there is a risk of damaging the optical elements or smearing surfaces of the optical elements when placing the polymer waveguide film in position.
Furthermore, since the adhesive is filled into the gap between the polymer waveguide film in which the waveguide is provided and the submount carrying the optical elements, the distance between the waveguide and the optical elements might increase, potentially causing a reduction in optical coupling efficiency.